• Best, Adam

Abstract

The Lithium oxygen (Li-O2) battery, which is proposed to use lithium metal as the anode and incorporate O2 into the cathode, exhibits a theoretical energy density of ~ 3500 Wh kg-1 [1]. This is a 10-fold increase in stored gravimetric energy density compared to traditional lithium-ion batteries, which makes the Li-O2 battery a promising candidate for next generation high energy storage devices. In simple terms, on discharge O2 is reduced to form the lithium peroxide (Li2O2) discharge products, and on charge, the Li2O2 is decomposed to release O2. However, the redox reaction mechanism in the Li-O2 battery is in reality more complicated. As shown in Fig 1, different pathways have been proposed for the discharge-charge process including the surface-mediated and solution-mediated mechanism. In the surface-mediated process when discharging, oxygen is first reduced to form lithium superoxide (LiO2), and then is further reduced to form the final Li2O2 product on the cathode surface. During the charging process, a lithium-oxide intermediate is formed from the cathode Li2O2 bound product, which decomposes to lithium ions and releases O2. On the other hand, during the solution-mediated discharging process, the Li2O2 product dissolves in solution and during the charging process, the formed intermediate LiO2 diffuses in solution then undergoes a disproportionation reaction to release Li+ and O2 into the solution.When examining the discharge-charge profile of the standard Li-O2 cell, two main challenges including the poor round-trip efficiency due to the high charge overpotential and limited capacity because of the passivation of the porous cathode are experienced as shown in Fig 1 (b). The electrolyte plays an important role in determining the performance of Li–O2 batteries, crucially influencing the formation and decomposition mechanism of solid discharge products Li2O2. An optimal electrolyte for Li–O2 batteries must meet several criteria: high O2 solubility and diffusivity, robust chemical and electrochemical stability under O2-rich conditions, effective coordination with intermediates to facilitate the solution-mediated mechanism, and a pronounced solvating effect to aid in the decomposition of Li2O2.This project aims to create a standardized air-breathing cell testing system for the investigation of Li-O2 battery, to study the fundamental mechanisms underlying the redox reactions within the electrolyte. Through these fundamental studies, we aim to address and overcome the specific challenges associated with electrolytes in Lithium-oxygen batteries.

Topics

• porous
• density
• impedance spectroscopy
• surface
• energy density
• Oxygen
• Lithium
• diffusivity
• decomposition